Protein molecules are essential for the functioning of every cell in the body. The body synthesizes some proteins foods we eat. Proteins are long chains of amino acids that form the basis of all life. Proteins are made up of hundreds or thousands of smaller units called amino acids, which are attached to one another in long chains. There are 20 different types of amino acids that can be combined to make a protein. These proteins bind and carry atoms and small molecules within cells and throughout the body.